An IC card system is widely known as a system for near-field communications. For example, the IC card system disclosed in JP 2010-200061 A comprises, as shown in FIG. 24, a reader/writer 280, which may be called “antenna apparatus,” and an IC card 285 acting as a transponder. The antenna apparatus 280 is an unbalanced circuit, which comprises a semiconductor 70, a noise filter (first filter) 71, a matching circuit 72, a low-pass filter (second filter) 73, and an antenna resonance circuit 66. The semiconductor 70 comprises a transmission circuit, a reception circuit, a modulation circuit, a demodulation circuit, a controller, etc. The antenna resonance circuit 66 comprises a near-field communication antenna 1a, a resistor (not shown), and a resonance capacitor 65. The resonance frequency of the antenna resonance circuit 66 is set to an inherent frequency (for example, 13.56 MHz) used for communications, at which the real part of the impedance of the antenna resonance circuit 66 is substantially in a short-circuited state. The antenna resonance circuit 66 is connected to the semiconductor 70 via the impedance-matching circuit 72.
An output terminal Tx connected to the modulation circuit in the transmission circuit in the semiconductor 70 is connected to the impedance-matching circuit 72 via the first filter 71 for EMC. An input terminal Rx connected to the demodulation circuit in the reception circuit in the semiconductor 70 is connected to a connecting point of the first filter 71 and the impedance-matching circuit 72 via the second filter 73 comprising a capacitor connected in series to a resistor.
The operation of the transmission circuit and the reception circuit is controlled by the controller. Signals having a frequency (for example, 13.56 MHz) corresponding to the tuning frequency is given from the oscillator to the transmission circuit, modulated by a predetermined protocol, and supplied to the antenna resonance circuit 66. The near-field communication antenna 1a of the antenna resonance circuit 66 is magnetically coupled to the near-field communication antenna 1b of the IC card 285 at a predetermined coupling coefficient. Accordingly, when the IC card 285 as a transponder is placed close to the near-field communication antenna 1a, the near-field communication antenna 1b in the IC card 285 is magnetically coupled to the near-field communication antenna 1a, resulting in electric power transmission to the integrated circuit 68 of the IC card 285 by electromagnetic induction, and data transmission according to a predetermined protocol (for example, ISO 14443, 15693, 18092, etc.).
As a near-field communication antenna (simply called “antenna” below) used for such system, JP 2005-094737 A proposes an antenna 200 comprising, as shown in FIG. 25, a single-flange, soft-magnetic core comprising a cylindrical body 351 and a rectangular, planar flange 355, and a coil 352 wound around the body 351, conductor wire ends 353, 353 of the coil 352 being connected to terminals 356, 356 disposed on the side and bottom surfaces of the flange 355, and a non-magnetic body having a flat upper surface (not shown) being disposed on the coil 352 and the body 351. Because magnetic flux generated by the coil 352 predominantly passes through the soft-magnetic core, a high-density magnetic flux is directed upward from the coil 352.
A wireless communications apparatus such as a mobile phone, etc. comprises, in a casing having a limited size, pluralities of antennas such as a main antenna mostly used for oral communications, a near-field communication antenna, an inductive charging antenna, a digital TV antenna, etc., together with other circuit devices. Accordingly, a low-height, near-field communication antenna with a small mounting area is demanded.
Even miniaturized near-field communication antennas are practically required to have communication distance of 30 mm or more. The miniaturization of an antenna requires the size reduction of a soft-magnetic member, but a small soft-magnetic member provides a reduced magnetic flux, resulting in difficulty to have a predetermined communication distance, and thus failing to carry out stable communications.
The self-resonance frequency of an antenna needs to be sufficiently higher than 13.56 MHz, a communication frequency band, generally recommended to be 40 MHz or more. In the antenna of JP 2005-094737 A, the coil 352 directly wound around the soft-magnetic body 351 has an inside region substantially filled with the soft-magnetic member, resulting in large self-inductance, providing the antenna with a low self-resonance frequency, which is near a communication frequency band. Such an antenna tends to have uneven self-inductance in a communication frequency band, because of the uneven parasitic reactance of the coil 352 and the uneven magnetic properties of the soft-magnetic member. Uneven self-inductance needs the adjustment of matching conditions of each antenna with other circuits, taking a lot of time and a many number of steps, thus resulting in cost increase of the antenna. Though self-inductance can be made smaller by providing the coil with a reduced number of turns or a reduced diameter, the coil generates less magnetic flux, resulting in a shorter communication distance.
To have enough communication distance, it may be considered to increase a magnetic field generated from the antenna with increased electric power supplied, but too much electric power supply magnetically saturates a magnetic body, or induces large electric power in a near mating antenna, likely destroying the semiconductor. To prevent it, a protecting circuit may be disposed, but it increases the number of parts.
It is possible to reduce the size of the soft-magnetic core disclosed in JP 2005-094737 A by using a soft-magnetic alloy having excellent magnetic properties such as permeability, a saturation magnetic flux density, core loss, etc., but it needs an insulating coating because the coil is directly wound around the soft-magnetic core. This increases the number of steps, resulting in higher production cost of the antenna. A resin member for protecting the coil 352 is also needed, resulting in a higher production cost.